Field of the Invention
The invention relates to an optical record carrier for writing and reading information by means of a radiation beam incident on an entrance face of the record carrier, comprising a substrate carrying a stack of layers, which stack comprises a recording layer, an interference layer and a reflection layer, in that order. An interference layer is a layer giving rise to optical interference effects.
3. Description of the Related Art
Many known optical record carriers such as the Compact Disc (CD), Compact Disc Read Only Memory (CD-ROM) and Laser Vision (LV) are pre-recorded by the manufacturer and can only be read by the user. Another type of record carrier, which is inscribable by means of a special write/read device and is also readable with the same read device as the one with which the aforesaid uninscribable record carriers are read, is also in general use. Examples are distribution of information in the form of audio or digital data to a small number of clients, for which the manufacture of a normal CD or CD-ROM is too expensive. However, for readability, the inscribed record carrier must satisfy the same standard requirements as the non-inscribable record carrier. This calls for an initial reflection of 70% or more and a contrast of 60% for the CD. The initial reflection is the reflection of an uninscribed part of the record carrier, while the contrast is the difference in reflection of the non-inscribed parts and the inscribed parts, divided by the initial reflection. It is found to be difficult to manufacture an inscribable record carrier which meets these requirements. This is particularly the case if materials referred to as phase-change materials are used for the recording layer. The crystal phase of these materials changes in response to irradiation with a laser beam, so that the reflection of the material changes. However, the known phase-change materials have too low reflection to be used without any difficulty in a record carrier which is compatible with the normal CD.
An inscribable record carrier of the type described above is known from European Patent Application No. 0 352 105. This record carrier comprises a substrate with the entrance face on one side and on the other side first a recording layer, subsequently an interference layer and finally a reflection layer. The radiation beam incident through the substrate is first incident on the recording layer and subsequently on the interference and reflection layers. In other words, the recording layer of the stack faces the entrance face. This stack increases the initial reflection as well as the contrast of the record carrier due to interference effects in the layers. The preferred embodiment of the record carrier described in said Patent Application has a recording layer of the phase-change material InSb; the record carrier has a reflection of 72% and 38% for the uninscribed and inscribed parts, respectively, hence a contrast of 47%.
A drawback of such known record carrier is that the structure of the stack leads to a reasonable increase of the reflection and the contrast for only a few recording materials. Other recording materials, having better write or erase properties, do not yield the desired high reflection and contrast with this stack. Increased contrast is not only desirable for a record carrier in order to meet the requirements of an industrial standard, but also for obtaining a higher signal-to-noise ratio so that the record carrier will be better readable.